1. Field of Invention
This invention relates to a photonic integrated circuit, specifically to a reflective arrayed waveguide grating, that is used for wavelength division multiplexing and demultiplexing for fiberoptic communication networks. It also relates to several other passive and active photonic device applications in the capacity of a building block via triple-phase integration described herein.
2. Description of the Prior Art
Recently arrayed waveguide gratings (AWGs) have proven to be an attractive vehicle for achieving high channel count wavelength division multiplexing (WDM) (see for example “An N×N optical multiplexer using a planar arrangement of two star couplers” disclosed by C. Dragone in IEEE Photonic Technology Letters, vol. 3, no. 9, 1991, pp 812815, or U.S. Pat. No. 0,033,715 A1 titled “Arrayed waveguide grating having a reflective input coupling,” Delisle, et al., Oct. 25, 2001). The basic strength and cost competitive success of this technology stems from the fact that it is built upon the strength of matured semiconductor fabrication (fab) technology. Many basic and precision fab facilities and advantages can be utilized in fabricating photonic integrated circuits (PICs) on silicon wafer or on other suitable wafers. The photonic components built on the surface of a wafer are generally known as the planar components; examples include planar waveguide, AWGs, interleavers, star couplers, variable optical attenuators (VOAs) and other integrated solutions that can be built around PIC chips. The planar star coupler, in fact, is the mother device for AWGs, because, in an AWG, two modified planar couplers are connected via an array of planar waveguides having a fixed path difference between successive ones, thereby acting as a grating. An analogy to the electronic ICs may be used to elucidate the photonic waveguides and PICs: PICs are the counterparts of the electronic ICs where photonic waveguides are the basic building blocks similar to the transistors for the ICs. Like transistors can perform switching, amplifying, and signal processing functions of electronic signal, photonic waveguides and photonic integrated circuits can be designed to perform similar functions with light signals.
Detailed construction of a regular AWG is illustrated in FIG. 1, which for the ease of comparison, will be termed as a transmissive arrayed waveguide grating (TAWG) because it functions by transmitting light from the input through the device to the output. A TAWG is commonly used as a multiplexer (MUX) and/or a demultiplexer (DMUX). As shown in FIG. 1, a TAWG is built on a substrate 10 on which waveguides and slabs are fabricated whose functionalities are described in the following. A TAWG has the following main functional parts: a single or plurality of input waveguides 3, an input slab 4, an array of waveguides containing plurality of neighboring waveguides 5, an output slab 6, and a plurality of output waveguides 7 whose number typically equals to the number of channels of the device.
Referring to FIG. 1, the functionality of a TAWG can be briefly explained as follows. The input waveguide 3 carries a multiplexed signal to the input slab 4 that couples the signal to the array of waveguides 5. The waveguides in the array are fabricated with a constant path difference, ΔL, between the neighbors. These waveguides lead the multiplexed signal to the output slab 6. Because of the path difference between successive waveguides, the light undergoes interference; the intensity of constructive interference is focused at well-defined positions at the other end of the output slab 6A. Here, output waveguides 7 are fabricated at calculated positions to collect the signals that are already separated into constituent wavelengths by the interference mechanism, thus completing the demultiplexing function. The multiplexing is achieved by following a reverse path: since the AWG works as a spectrograph device, when individual wavelengths (signals) are launched into the waveguides 7 attached to the output slab 6, they will form a combined or “multiplexed” signal at the end of the input slab 4. Thus a TAWG (e.g., the one shown in FIG. 1) functions by means of transmitting light from the input terminal via the input waveguide through the entire device.
The structure as a whole 11 referred to as an optical chip or a photonic integrated circuit (PIC), because it is constructed by assembling the waveguide elements in an integrated fashion. While in the current practice of PICs it is not implemented yet, this inventor envisions that a PIC may also contain additional elements such as modulator, amplifier and detector on the same substrate, thus qualifying the PIC as a platform for designing many passive and active photonic devices via triple-phase integration, as described in the preferred embodiments.
The TAWG described above suffers from several disadvantages such as bigger area per device, longer waveguide lengths in the array, and two different slabs for input and output, all of which contribute to a higher insertion loss. Two different fiber-array interfaces are required for packaging; one for input and one for output. Having two different external interfaces is also disadvantageous, because, attaching two different fiber arrays increases the packaging loss, makes it relatively less reliable, and increases chance of failure.
AWG Size Reduction
A compact design of AWGs has been the subject matter of many contemporary investigations. Here we review available designs as published in the literature with a view to contrast the present invention with the previous attempts.
A design titled “Silica-based arrayed waveguide grating circuits as optical splitter/router,” published in Electronics Letters, vol. 31, No. 9, 1995, pp 726–727, by Y. Inoue, A. Himeno, K. Moriwaki and M. Kawachi, proposed a reflection type arrayed waveguide grating circuit for optical power splitting and wavelength routing functions. The authors reported 1×14 optical power splitter with a mean insertion loss of 15.5 dB and the wavelength router's insertion loss ranged from 5.2 dB to 8.5 dB (a channel non-uniformity of 3.3 dB) with an inter-channel crosstalk of 19 dB.
Polarization insensitive, InP based DMUX was demonstrated for a 1×16 device [see “Small-size, polarization independent phased-array demultiplexers on InP,” by H. Bissessur, B. Martin, R. Mestric and F. Gaborit, published in Electronics Letters, vol. 31, No. 24, 1995, pp 2118–2120] that exhibited an insertion loss of 8 dB and the crosstalk was 14 dB.
A design for two-slab waveguide grating routers via InP/InGaAsP material that would reduce the chip size was reported (see “Compact design waveguide grating routers,” by T. Brenner, C. H. Joyner, and M. Zirngibl, published in Electronics Letters, vol. 32, no. 18, 1996, pp 1660–1661). This design is based on a pair of interlaced conventional waveguide grating routers that share the slab area by means of a cleavage through the centers of the shared slabs (or free space region, in their nomenclature). The reported on-chip transmission loss was 7.5 dB for the central channel and 1 dB higher for the outermost ports of the 8-channel device. The reported crosstalk was on the order of 20 dB.
InP based waveguide grating router's size reduction by folding back the input/output ports and exploiting mirror reflections off of multiple facets created by cleaves was reported (see “Size reduction of waveguide grating router through folding back the input/output fanouts,” by M. Zirngibl, C. H. Joyner, and J. C. Centanni, published in Electronics Letters, vol. 33, No. 4, 1997, pp 295–297). They reported an insertion loss of 4 dB for the best channel measured against a straight test waveguide and 2–3 dB more for triple cleaved router. No crosstalk number was reported.
A reflective waveguide array demultiplexer using electro-optic LiNbO3 was proposed (see “Reflective waveguide array demultiplexer in LiNbO3,” by H. Okayama, M. Kawahara, and T. Kamijoh, in Journal of Lightwave Technology, vol. 14, No. 6, 1996, pp 985–990). For a 1×4 device, the authors have reported an insertion loss of 15 dB and crosstalk ranging from 12 to 25 dB.
It is clear from the above review that previous attempts of the chip's size reduction of several photonic integrated circuits have enjoyed a partial success. In all cases, however, size reduction is accompanied by a higher loss and lower performance. The present invention addresses the chip size reduction with simultaneous improvement of the device performance, and outlines methods of using the RAWG as a building block to fabricate additional devices via triple-phase integration.